The wave particle duality principle of quantum physics holds that matter and light exhibit the behaviors of both waves and particles, depending upon the circumstances of the experiment. It is a complex topic, but among the most intriguing in physics. Wave Particle Duality in LightIn the 1600s, Christiaan Huygens and Isaac Newton proposed competing theories for light's behavior. Huygens proposed a wave theory of light while Newton's was a "corpuscular" (particle) theory of light. Huygens' theory had some issues in matching observation. Newton's prestige helped lend support to his theory, so for over a century his theory was dominant. Light functions as both a particle and a wave, depending on how the experiment is conducted and when observations are made. Wave Particle Duality in MatterThe question of whether such duality also showed up in matter was tackled by the bold de Broglie hypothesis, which extended Einstein's work to relate the observed wavelength of matter to its momentum. Experiments confirmed the hypothesis in 1927, resulting in a 1929 Nobel Prize for de Broglie. Significance of Wave Particle DualityThe major significance of the wave particle duality is that all behavior of light and matter can be explained through the use of a differential equation which represents a wave function, generally in the form of the Schrodinger equation. This ability to describe reality in the form of waves is at the heart of quantum mechanics.

• The photoelectric effect posed a significant challenge to the study of optics in the latter portion of the 1800s. It challenged the classical wave theory of light, which was the prevailing theory of the time. It was the solution to this physics dilemma that catapulted Einstein into prominence in the physics community, ultimately earning him the 1921 Nobel Prize.

• What is the Photoelectric Effect?• Though originally observed in 1839, the photoelectric

effect was documented by Heinrich Hertz in 1887 in a paper to the Annalen der Physik. It was originally called the Hertz effect, in fact, though this name fell out of use.

Light collides with a metal surface, releasing electrons.

Photoelectric Effect• The photoelectric effect is a phenomenon in which electrons are emitted

from matter after the absorption of energy from electromagnetic radiation .• The emitted electrons can be referred to as photoelectrons.• The photoelectric effect refers to the emission, or ejection, of electrons

from the surface of, generally, a metal in response to incident light.

Energy contained within the incident light is absorbed by electrons within the metal, giving the electrons sufficient energy to be 'knocked' out of, that is, emitted from, the surface of the metal.

Using the classical Maxwell wave theory of light, the more intense the incident light the greater the energy with which the electrons should be ejected from the metal. That is, the average energy carried by an ejected (photoelectric) electron should increase with the intensity of the incident light.

In fact, Lénard found that this was not so. Rather, he found the energies of the emitted electrons to be independent of the intensity of the incident radiation.

• Actually Einstein found our the energy is tied to the frequency of the light.• E = hf

• What Is Quantum Physics?: • Quantum physics is the study of the behavior of matter and energy

at the molecular, atomic, nuclear, and even smaller microscopic levels. In the early 20th century, it was discovered that the laws that govern macroscopic objects do not function the same in such small realms.

• What Does Quantum Mean?: • "Quantum" comes from the Latin meaning "how much." It refers to

the discrete units of matter and energy that are predicted by and observed in quantum physics. Even space and time, which appear to be extremely continuous, have smallest possible values.

• Who Developed Quantum Mechanics?: • As scientists gained the technology to measure with greater

precision, strange phenomena was observed. The birth of quantum physics is attributed to Max Planck's 1900 paper on blackbody radiation. Development of the field was done by Max Planck, Albert Einstein, Niels Bohr, Werner Heisenberg, Erwin Schroedinger, and many others. Ironically, Albert Einstein had serious theoretical issues with quantum mechanics and tried for many years to disprove or modify it.

• What's Special About Quantum Physics?: • In the realm of quantum physics, observing

something actually influences the physical processes taking place. Light waves act like particles and particles act like waves (called wave particle duality). Matter can go from one spot to another without moving through the intervening space (called quantum tunnelling). Information moves instantly across vast distances. In fact, in quantum mechanics we discover that the entire universe is actually a series of probabilities. Fortunately, it breaks down when dealing with large objects, as demonstrated by the Schroedinger's Cat thought experiment.

Schrodinger's cat• Schrödinger's cat is a famous illustration of the principle in quantum theory of

superposition , proposed by Erwin Schrödinger in 1935. Schrödinger's cat serves to demonstrate the apparent conflict between what quantum theory tells us is true about the nature and behavior of matter on the microscopic level and what we observe to be true about the nature and behavior of matter on the macroscopic level.

• Here's Schrödinger's (theoretical) experiment: We place a living cat into a steel chamber, along with a device containing a vial of hydrocyanic acid. There is, in the chamber, a very small amount of a radioactive substance. If even a single atom of the substance decays during the test period, a relay mechanism will trip a hammer, which will, in turn, break the vial and kill the cat. The observer cannot know whether or not an atom of the substance has decayed, and consequently, cannot know whether the vial has been broken, the hydrocyanic acid released, and the cat killed. Since we cannot know, the cat is both dead and alive according to quantum law, in a superposition of states. It is only when we break open the box and learn the condition of the cat that the superposition is lost, and the cat becomes one or the other (dead or alive). This situation is sometimes called quantum indeterminacy or the observer's paradox : the observation or measurement itself affects an outcome, so that the outcome as such does not exist unless the measurement is made. (That is, there is no single outcome unless it is observed.)

• We know that superposition actually occurs at the subatomic level, because there are observable effects of interference, in which a single particle is demonstrated to be in multiple locations simultaneously. What that fact implies about the nature of reality on the observable level (cats, for example, as opposed to electrons) is one of the stickiest areas of quantum physics. Schrödinger himself is rumored to have said, later in life, that he wished he had never met that cat.

Quantum Theory

• Discrete bundles of energy are called quanta. The energy of the quanta is directly proportional to the frequency.

• E = hf

• h is called Plank’s Constant 6.63 x 10-34 (J)(s)

• 1 eV = 1.60 x 10-19 J

• The quantum of energy is called a photon.

• Ephoton = hf = hc/λ

• The equation states that the energy of a photon is directly proportional to its frequency and inversely proportional to its wavelength.

• Photons of light are released or absorbed in exact amounts. Although a photon is a massless particle of light (electromagnetic spectrum), it carries both energy and momentum.

DeBroglie’s wavelength

• λ = h/p

• DeBroglie stated that all matter has wave like properties. The momentum of the object is inversely proportional to the wavelength.